1. Field of the Invention
This invention relates to cements used for current collector bar-carbon block joints of electrolytic reduction cells using molten salt electrolytes, e.g. those used for the production of aluminum.
2. Discussion of the Prior Art
Aluminum is conventionally produced by the reduction of alumina in a "Hall-Heroult" electrolytic cell provided with a lining made of prebaked carbon blocks. The lining acts both on a refractory material to protect the cell walls and bottom from the hot molten electrolyte and aluminum, and as a cathode for the electrolysis process. Current is conveyed from the carbon lining by steel collector bars which extend into slots in the carbon blocks. The slots are made slightly larger than the collector bars to allow for ease of assembly, different rates of expansion of the steel and carbon and slight movements of the collector bars. However, the electrical connection between the carbon lining blocks and the steel collector bars must be good, so an electrically conductive material is generally used to fill the free space between the bars and the carbon blocks.
The bar-block joint material is conventionally either cast iron rodding or a conductive cement, but both these materials are found to be undesirable in certain respects. The use of cast iron rodding is expensive and many installations are not equipped for its use.
The conductive cement used in the bar-block joints is traditionally a hot ramming mix consisting of a carbonaceous aggregate, such as calcined anthracite, and a binder, such as pitch or a tar-pitch mixture. The mix is tamped into the joint at a temperature of about 100.degree. C. to 130.degree. C. The use of pitch or tar-pitch mixtures as a binder causes environmental problems. During the filling of the joints, workers are exposed to tar fumes from the hot mix and to noise generated by tamping tools.
Attempts have been made to overcome these problems by providing room temperature cements. However, these cements must be capable of maintaining a good electrical contact between the collector bars and the carbon blocks and of forming a cathode assembly having no cracks or gaps since the cathode assembly serves as a container for the molten bath and metal. We have found that commercially available room temperature cements are not very satisfactory because they form joints having low conductivities or cause the blocks to crack at the high operating temperatures of electrolytic cells.